Toshitaka Oka

Interlaboratory comparison of positron annihilation lifetime measurements for synthetic fused silica and polycarbonate

Interlaboratory comparison of positron annihilation lifetime measurements using synthetic fused silica and polycarbonate was conducted with the participation of 12 laboratories. By regulating procedures for the measurement and data analysis the uncertainties of the positron lifetimes obtained at different laboratories were significantly reduced in comparison with those reported in the past.

Positronium annihilation and pore surface chemistry in mesoporous silica films

Lifetimes of ortho-positronium in mesoporous silica films were measured before and after surface trimethylsilylation of –OH groups. Variations of positronium lifetimes in the mesopores upon the surface modification indicate that the interaction between positronium and the pore surface is weakened in the pores, whose surface is covered with –CH3 groups, in comparison with those covered with –OH groups. This is consistent with the authors’ previous observation that positronium slowing down is less efficient in the pores covered with –CH3 groups. The present work demonstrates that in the porosimetric application of positron annihilation lifetime spectroscopy, the interaction between positronium and the pore surface has to be properly taken into consideration.

Free volume expansion and nanofoaming of supercritical carbon dioxide treated polystyrene.

Free volume of polystyrene films treated with supercritical carbon dioxide (SCCO 2) were examined by positron annihilation lifetime spectroscopy. Variation of the free volume sizes after the SCCO 2 treatment due to the release of trapped CO 2 and structural relaxation of the polymer was observed. After 500 h from the depressurization, the free volume of the treated films free from CO 2 was approximately 0.306 nm in radius and much larger than that of the untreated films of approximately 0.295 nm in radius due to the freezing of the swollen structure caused by the large solubility of CO 2.

Free volume change of elongated polyethylene films studied using a positron probe microanalyzer

Free volume change of low density polyethylene (LDPE) and high density polyethylene (HDPE) films upon mechanical deformation was microscopically investigated by positron probe microanalysis (PPMA). The ortho-positronium (o-Ps) lifetimes were gradually shortened by uniaxial deformation, indicative of shrinkage of the free volume. The o-Ps intensity for HDPE increased by deformation, whereas that for LDPE varied little. It suggests that destruction of crystallites plays an important role in the deformation of HDPE. PPMA is demonstrated to be a promising, powerful probe investigating free volume changes, at different local points subjected to different degrees of deformation, in elongated polymers.

The local reaction processes of polyethylene irradiated with heavy ion-beam

The local reaction induced in the track for low-density polyethylene irradiated with ion-beam was investigated. The transformations of the chemical structure induced by irradiation are on trans-vinylene, end-vinyl, vinylidine and the cross-link. The different reaction processes are induced in the micro-region of the track. These processes depend on the stopping power. In the region of larger stopping power, the rate of the process, which produces the end-vinyl, increases and that of the trans-vinylene decreases. These results suggest that the main chain scission of the polyethylene can be induced by very large energy deposition in the micro-region.

Long-term degradation of chemical structures and mechanical properties in polyethylene induced by ion-beam irradiation

The long-term oxidative degradation of chemical structures and the mechanical properties in different polyethylenes induced by ion-beam irradiation were investigated. They were measured along the ion trace when the samples were stored in the atmosphere for a very long time. The hydroperoxide group is very important in the degradation process. These degradation processes are attributed to the mobility of oxygen molecule due to the density and the crystallinity of polyethylene.